The matrix of cartilage has been shown to contain a number of proteins that are important as a safeguard against enzymic destruction. These proteins have been previously isolated and termed Anti-Invasion Factor (AIF). This mixture of non- collagenous and non-proteoglycan proteins contains protease inhibitors which act to protect the matrix against enzymic destruction in vivo. Originally, these components were used to study the inhibition of enzymic degradation of cartilage by invading tumor cells, and a variety of protease inhibitors against tumor cell proteases were isolated in the past. It is a well-known fact that during inflammatory episodes leucocytes infiltrate the joint capsule and by releasing their enzymes, especially elastase, participate in the breakdown of the cartilagenous matrix components. Since elastease is a potent proteolytic enzyme which can degrade proteoglycans, collagens and probably other non-collagenous matrix components, its regulation (and inhibition) must be understood.